1. Field of the Invention
The invention relates to a method and a device for in vivo detection of the direction of Langer""s lines in the skin.
2. Description of the Prior Art
Langer""s lines (Sulci cutis) define the direction within the human skin along which the skin is least flexible. This mechanical property is determined by the alignment of collagen fibers and bundles of collagen fibers within the dermis.
The fundamental research on this property of the human skin was carried out by K. Langer and was already published in 1861. To determine the direction of the Langer""s lines, he multiply pierced human cadaver skin at short distances. Although his piercing instrument was of circular shape, the resulting holes were of ellipsoidal shape. Langer observed patterns of the directions of the longer axes of the ellipsoidal holes on a skin area. Subsequently, these patterns were given the name xe2x80x9cLanger""s linesxe2x80x9d according to it""s discoverer.
The knowledge of the direction of Langer""s lines within a particular area of the skin is of great importance for surgical operations. Generally and most importantly, a surgical cut should allow for an optimal opening of the area to be operated on and should also offer the possibility to extend the area during the surgery. At the same time, it must be made sure that the skin can heal properly after the surgery and that a beneficial cosmetic appearance is obtained after healing. These conditions are usually best fulfilled, if a surgical cut is carried out in the direction of Langer""s lines. The generation of scares, in particular, is minimized under these conditions. This is of paramount importance in plastic surgery, where surgical cuts and potential scars run through visible body parts, such as the face.
Unfortunately, the accurate direction of Langer""s lines often is not known. In some areas of the body, large differences exist in the direction between different persons. Even on the same person, changes in the exact direction may occur during the course of life. To account for these variations in the direction of Langer""s lines during surgery, it is necessary to determine the direction using a non invasive method. With such a method, even less experienced surgeons would be enabled to plan a surgical cut with minimal scaring. This could reduce the esthetic and psychological problems associated with large scars and eliminate the need for post-operative treatment of scaring.
Despite the large importance of this problem, a suitable method for non-invasive measurement of Langer""s lines is not yet available. In the past it was attempted to use mechanical tension measurements on the skin surface as e.g. described by J. C. Barbenell in an article xe2x80x9cIdentification of Langer""s Linesxe2x80x9d, pp. 341-344 in: Handbook of Non-Invasive Methods and the Skin, CRC Press, Boca Raton, 1995. In this publication, it becomes obvious that the described mechanical method does not give reliable information about the direction of Langer""s lines in the skin.
The goal of the invention is to measure the direction and the pattern of directions of Langer""s lines on the skin surface non invasively and painlessly.
This problem is solved using light penetrating the skin. Light is irradiated as primary light into the skin at a defined site on the skin surface, in such a way that the light is transported in the skin by scattering and absorption. Part of said irradiated light emerges from the skin as secondary light in the region surrounding the irradiation site. As a measure of the direction of Langer""s lines, the dependence of a measurable property of the secondary light is measured as a function of the polar angle around the irradiation site and the preferential direction of light transport in the skin is determined. This preferential direction of light transport indicates the direction of Langer""s lines.
A measurable property of the light (subsequently also termed xe2x80x9cmeasurement parameterxe2x80x9d) is, particularly, the intensity of light. Other measurable properties of the light can also be used to determine the preferential direction of the diffuse light transport in the skin. E.g. the intensity of the primary light can be modulated. In this case, the AC amplitude (modulation) of the measured secondary light can be used to obtain information about the preferential direction of the diffuse light transport. When polarized light is used as primary light, the degree of polarization of the secondary light may be used. Generally, any measurable property of the secondary light, which contains information about the preferential direction of the diffuse light transport in the skin can be used. Subsequently, it is referred to the intensity as the measurement parameter as an example without restricting the use to using the intensity as the measurement parameter.
To determine the polar angle dependence, the primary light is irradiated into the skin within a small, specially confined area (irradiation site). A requirement for the determination of the polar angle dependence around the irradiated area is the measurement of the intensity of the secondary light on at least two detection sites. The polar angle (with respect to the irradiation area) of the at least two detection sites must have an orthogonal component. If only two detection sites are used, a difference in the polar angle of 90xc2x0 is preferred, whereas the difference in the polar angle should be at least 35xc2x0.
At least three detection sites should be used to achieve an adequate resolution of the measured polar angle dependence of the measurable property. The difference in polar angle of the measurement locations should preferably be smaller than 20xc2x0. Especially preferred is the measurement of the measurement parameter at a multitude of locations located around the irradiated area, for which an angular resolution of at least 20xc2x0 is preferred, i.e. the difference in polar angle between two neighboring measurement locations is not larger than 20xc2x0.
The wavelength of the primary light is preferably between 400 nm and 1400 nm. It is furthermore preferred, with respect to the accuracy of the measurement, that the light is predominantly monochromatic. It is sufficient, if the maximum half width is smaller than 200 nm, preferably smaller than 100 nm. Furthermore, the accurate determination of the preferential direction of light transport and therefore the determination of the direction of Langer""s lines may be improved by using polarized light as primary light. Under certain conditions it may also be helpful to place a polarization filter between detection area and light detector.
Different detection sites can be obtained by using flexible detection means, e.g. light guiding fibers, which are moved from detection site to detection site. Especially preferred are embodiments, which incorporate a multitude of detection means to measure the polar angle dependence. In this embodiment, the individual detection means are positioned in a fixed location with respect to the irradiation site and measure the intensity of the secondary light with emerges from the skin at a defined detection site.
The detection sites, of which the measurement parameters are used to determine a polar angle dependence around the irradiated site, are preferably aligned on a circle around the irradiated site to ensure equal distances to the irradiated site. This eases the subsequent mathematical manipulation of the data. Furthermore, it was observed that the anisotropy for different distances between irradiation site and detection areas varies with the location of the human body. The reason for this effect is the dependence of the penetration depth of the light on the distance between irradiation area and detection area.
The measurement of the polar dependence of light intensity can generally be achieved with methods and means, which are known for other purposes, however, special requirements of the measurement need to be accounted for. WO 94/101 describes a method for measurement of glucose in skin, for which the analytical result is determined from a number of spacially dependent light scattering measurements. For these measurements, light is irradiated into the skin at a defined site on the skin (irradiated site) and the light intensity measured from a second defined site on the skin in the surrounding area (detection area) of the irradiated site. A special, chess board like alignment of irradiating light sources (especially light emitting diodes) and light detectors (especially photo diodes) can be used in particular for this purpose. A very suitable apparatus for such measurements and other measurements of light transport in the skin is described in the European Patent with the publication number 0777119. This apparatus uses an optical fiber plate (face plate) which is in contact with the skin and facilitates the transmission of light from the detection sites on the surface of the skin to the light detectors in the apparatus. In addition to the descriptions in this invention it is referred to these publications.
It is assumed in these references that the light transport in the skin, as governed by optical absorption and scattering, generally is isotropic in the skin surrounding a spacially limited irradiated site. Disturbances of the isotropic light diffusion are only expected to come from special heterogeneous structures in the skin, such as pigmented areas or blood vessels close to the skin surface.
A number of publications describe the detection of structures, particularly tumors, below the skin surface using light penetrating the skin. This is discussed e.g. in WO 96/04545 and the corresponding references. Again, in this publication it is assumed that the light transport in the skin is generally isotropic, apart from the heterogeneous structures the method wants to detect.
In contrast to the assumption of these referenced publications, it has been discovered that the light transport in the skin is surprisingly and significantly anisotropic when the special measurement situation is used as disclosed in this invention. Furthermore, it has been discovered that the direction of anisotropy of the light transport indicates the direction of Langer""s lines.
The invention will be explained in more detail below by means of the figures.
FIG. 1 shows an apparatus for the detection of the direction of Langer""s lines in a perspective view from one side.
FIG. 2 shows a cut through the apparatus according to FIG. 1, in which the electronic circuits are shown as a block diagram.
FIG. 3 shows a detailed view of a portion of the apparatus in FIG. 2, which is in direct contact with the skin.
FIG. 4 shows a surface view of the light detection unit along the line IV-V as shown in FIG. 3.
FIG. 5 the alignment of irradiation site and detection sites in an alternative embodiment of a light detection unit.
FIG. 6 a drawing of the principle to explain the use of an apparatus, according to the invention in abdominal region of the body.
FIG. 7 the usual direction and pattern of Langers lines in the same abdominal region as shown in FIG. 6.
FIG. 8 a polar angle diagram of the intensity distribution of light, which emerges from the skin around an irradiated site. Measured in the abdominal region of the body as shown in FIGS. 6 and 7.
FIG. 9 a polar angle diagram as shown in FIG. 8. Two different measurement distances between irradiation site and detection sites are shown.
FIG. 10 a drawing of the principle of an alternative embodiment with the display of Langer""s lines directions on the skin surface using laser projection.